1. Field of the Invention
The present invention generally relates to scanner elements and, more particularly, to a scanner element that deflects an optical beam by swinging a micro mirror, and an optical scanning apparatus and an image forming apparatus using an optical writing apparatus used for scanning an exposure light according to electrophotography.
2. Description of the Related Art
In recent years, an optical scanning apparatus, which scans an optical beam such as a laser light, is used in optical equipments such as a barcode reader, a laser printer, a head mount display, etc. As an optical scanner of this type, there is suggested an apparatus having a structure in which a micro mirror, which is formed by using a micro-machining technique, is swung.
FIGS. 1A through 1C show an optical scanner having a micro mirror which is formed by using a micro-machining technique. FIG. 1A is a perspective view of the optical scanner; FIG. 1B is a cross-sectional view of the optical scanner; and FIG. 1C is a plan view of the optical scanner. The optical scanner 10 comprises a substrate 2, which is formed by a dual layer substrate of SOI that is formed of two sheets of conductive silicon substrates 200 and 201 being applied to each other via an insulative silicon oxide film 202. A mirror 1 as a movable plate is supported using elastic members (torsion bars) 3 which serve as beams provided along a straight line. Movable electrodes 8 are provided in the mirror 1. Stationary electrodes 9 are provided in a frame part 5 so as to face the movable electrodes 8, respectively. The movable electrodes 8 and the stationary electrodes 9 are separated from each other by separation grooves 6. Additionally, bonding pads 4 for the stationary electrode 9 and bonding pads 7 for the movable electrodes 8 are connected to the stationary electrodes 9 and the movable electrodes 8, respectively, via the conductive silicon substrate 200. The optical scanner 10 causes the micro mirror 1 to reciprocally swing using the two elastic members 3 as a torsional rotation axis according to an electrostatic attraction force between the movable electrodes 8 and the stationary electrodes 9.
As a method of forming the above-mentioned micro mirror, forming a resist film on one side of the dual layer substrate of SOI using spin coating; thereafter, developing and fixing the resist film by using a photo mask to form the frame part 5, the micro mirror 1, the elastic members 3, the movable electrodes 8, the stationary electrodes 9 and the separation grooves 6 according to a photolithography; and etching the pattern to the silicon oxide film according to a dry-etching method using the patternized resist film.
Subsequently, a resist film is formed on an opposite side of the SOI substrate in the same manner, the resist film is patternized using a photo mask to left the frame part 5 and the stationary electrodes 9, and, thereafter, parts excluding the frame part 5 and the stationary electrodes 9 are etched to the surface of the silicon oxide film surface. Thereafter, the exposed parts of the silicon oxide film are removed by etching and the bonding pads 4 and 7 are formed on predetermined positions.
FIG. 2 is a perspective view of the optical scanner 10 and a stem 12 to which the optical scanner 10 is mounted. A counter-sink part 11 is formed in the stem 12 so that the swing of the micro mirror 1 is not prevented. FIG. 3 is a perspective view of the assembly of the optical scanner 10 and the stem 12.
Moreover, there is suggested a structure as an example of the optical scanning apparatus using the above-mentioned micro mirror in which a plurality of optical scanners are provided. FIG. 4 is an illustration of the optical scanning apparatus provided with a plurality of optical scanners. A plurality of optical scanners 102 are provided on a base 101. A drive device 103 supplies a drive current to each optical scanner 102. Laser light sources 104 are provided to the respective optical scanners 102. Laser beams emitted by the laser light sources 104 are reflected by the respective mirrors of the optical scanners 102 so that reflected laser beams 105 are obtained.
Moreover, the following laser printer is suggested as an image forming apparatus using the above-mentioned optical scanning apparatus. FIG. 5 is an illustration of a laser printer as an example of an image forming apparatus. The laser printer 106 as an image forming apparatus comprises: an optical scanning apparatus 107 having the above-mentioned structure; a photoconductor 108 on which a latent image is formed by a reflected laser beam which is deflected by a mirror of the optical scanning apparatus 107; a developer 109 for developing the electrostatic latent image formed on the photoconductor 108 to a toner image; a transfer unit 110 for transferring the toner image on the photoconductor 108 to a recording medium; a recording medium supplier 111 for supplying the recording medium to an image forming part; and a fixation unit 112 for fixing the toner image on the recording medium.
FIG. 6 is an illustration of the optical scanning apparatus 107 and the photoconductor 108 shown in FIG. 5. A plurality of optical scanners are arranged in a main scanning direction. The laser light sources 104 emit laser beams according to image signals generated by an image signal generating apparatus (not shown in the figure). The laser beams irradiated by the laser light sources 104 incident on the optical scanning apparatuses 107. The reflected laser beams 105 that are deflected by mirrors in the optical scanning apparatus 107 form a latent image on the photoconductor 108.
In the above-mentioned image forming apparatus, a variation in the resonance frequency of the optical scanners constituting the optical scanning apparatus may cause a problem. FIG. 7 is a graph showing results of the resonance frequency f0 of each optical scanner. Each optical scanner has its own resonance frequency f0 as shown in FIG. 7. The variation in the resonance frequency f0 occurs due to the following cause.
FIG. 8A is a perspective view of an optical scanner of which structure is simplified for the sake of easy description of the resonance frequency of the optical scanner. FIG. 8B is a cross-sectional view of the optical scanner shown in FIG. 8A. The optical scanner comprises: a mirror 1 supported by two elastic members 3 as beams provided along a straight line that provide a rotational axis; a movable electrode 8 provided in the mirror 1; and a stationary electrode 9 provided in a frame member 5 so as to face the movable electrode 8. The mirror 1 is reciprocally swung by an electrostatic attraction force between the movable electrode 8 and the stationary electrode 9 using the two elastic members 3 as a torsional rotation axis. In this optical scanner, the resonance frequency of the mirror 1 is given by the following equation (1).f0=½π√{square root over ( )} (K/I)  (1)
where I represents a moment of inertia of the mirror, and K represents a spring constant determined by the two elastic members (torsion bars).
As mentioned above, in order to form the above-mentioned small size mirror (micro mirror), it is generally performed to form an etching mask by photolithography so as to form the shape of the mirror and other parts by using etching using the etching mask. However, in the process of forming the mirror, a transfer variation and over-etching or under-etching may occur. The transfer variation occurs due to the resist pattern being larger or smaller relative to the photo mask pattern in an exposure and development process of the photo mask pattern. The over-etching or under-etching occurs due to an object to be processed by etching being larger or smaller relative to the resist pattern.
Thus, there is a variation occurring in the accuracy of processing the elastic members 3. Due to the processing error, the shape of the torsion bars may vary with respect to an originally designed shape, which results in a variation in the spring constant K of the torsion bars. Additionally, the shape of the mirror also varies due to the same processing error with respect to the designed shape, which causes a variation in the moment of inertia of the mirror.
Therefore, a variation may occur in the value of the resonance frequency f0 represented by the equation (1). In the optical scanning apparatus having a plurality of optical scanners, if there is a variation in the resonance frequency f0 among the mirrors of the optical scanners, a variation occurs in a swing angle of the mirrors shown in FIG. 8B.
FIG. 9 is a graph showing a relationship between variation in etching and the resonance frequency. The optical scanner used for measurement has a weight of about 1.5 mg and a moment of inertia of 2.3×10−5 erg·cm. The torsion bar has a cross-section of 0.09 mm×0.2 mm and a length of 2 mm. FIG. 9 shows a fluctuation in the resonance frequency due to a variation in etching when processing the optical scanner. It can be appreciated from the graph of FIG. 9 that a variation in etching of ±2 μm with respect to a design value causes the fluctuation in the resonance frequency of 140 Hz.
Therefore, in an image forming apparatus using the above-mentioned optical scanner, a connection part between images formed by adjacent optical scanners is visible, which causes a problem that an image quality is deteriorated. Thus, there has been suggested a method of adjusting a resonance frequency of an optical scanner.
FIG. 10 is a perspective view of an optical scanner which is capable of adjusting a resonance frequency thereof according to a first method (for example, refer to Patent Document 1). In the first method, a planer type galvanometer mirror having a movable plate provided with mass-loading parts 13 and 14 on opposite ends thereof is operated and a difference between an initial resonance frequency and a previously set target resonance frequency is read. Then, a number of pulses of a laser light to be irradiated is calculated in accordance with the difference, and the thus-determined laser light is irradiated onto the mass-loading parts 13 and 14 so as to reduce the mass of the galvanometer mirror to adjust the resonance frequency to the target value.
FIG. 11 is a perspective view of an optical scanner showing a second method of adjusting a resonance frequency (for example, refer to Patent Document 2). In the second method, comb-shaped parts 18 are provided at corners of the mirror 1 so as to adjust a moment of inertial of the mirror. A laser beam 15 of a CO2 laser deflected by a deflection mirror 16 is irradiated onto a root of one of the comb-shaped parts 18 so as to cut off the one of the comb-shaped parts 18. Each of the comb-shaped parts 18 has a small mass relative to the mirror 1, and, thus, a moment of inertia of the mirror is reduced in response to a number of the comb-shaped parts 18 that are removed from the mirror. Accordingly, a quantitative adjustment of the resonance frequency of the mirror can be made.
Furthermore, there is a third method of adjusting a resonance frequency by controlling an amount of deposition of metal to a mirror or an amount of etching of the mirror in a manufacturing process of the galvanometer mirror (for example, refer to Patent Document 3).
[Patent Document 1] Japanese Laid-Open Patent Application No. 2002-40355
[Patent Document 2] Japanese Laid-Open Patent Application No. 2003-84226
[Patent Document 3] Japanese Laid-Open Patent Application No. 2002-40353
According to the above-mentioned first and second methods, a high-power laser apparatus is needed to adjust the frequency of the mirror, which causes a problem in that a manufacturing cost is increased. Additionally, there is a problem in that substances scattered by laser irradiation may adhere to a surface of the mirror, which causes a pollution of the mirror. Further, since a high-power laser is irradiated in the vicinity of the mirror surface, it is difficult to prevent temperature rise of the mirror component. A local temperature rise in the mirror component may cause distortion of the mirror component. Additionally, since frequency adjusting process must be performed on an individual device basis, a number of processes is increased, which results in an increase in a manufacturing cost.
On the other hand, according to metal vapor deposition of the etching according to the above-mentioned third method, it is difficult to control a distribution of an amount of vapor deposition or an amount of etching in a semiconductor wafer when a plurality of galvanometer mirror (optical scanner) are formed on the semiconductor wafer by using a semiconductor manufacturing technology. Thus, it is difficult to optimize the resonance frequency of each of the galvanometer mirrors on the semiconductor wafer.